Recombinator for eliminating hydrogen from accident atmospheres

ABSTRACT

The invention relates to a catalyst element for a recombinator for eliminating hydrogen from accident atmospheres, in which the technical problem of continuously efficiently converting both small and large amounts of hydrogen with the atmospheric air-oxygen present in the safety containers within a broad concentration range, and routing away the reaction heat arising in the process to such an extent that the respective ignition temperature is not reached in the present mixture is resolved by having the catalyst element exhibit a flat basic body ( 2 ), which is arranged within the area of flow through the recombinator, wherein the surface of the basic body ( 2 ) over which the accident atmosphere flows has a varying coverage density with catalyst material ( 3 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to devices that can be used to eliminate releasedor accidentally formed hydrogen from non-inerted spaces, e.g., safetycontainers of pressurized water reactors and non-inerted boiling waterreactors, which contain steam, air, aerosols and other gases in additionto hydrogen, effectively without backfiring. In this case, the hydrogencan be recombined into steam within the device in the presence of theexisting atmospheric oxygen, e.g., in a catalytic procedure.

2. Background Information

During the course of serious accidents, large amounts of hydrogen areformed in light-water reactors (LWR) due to the reduction of steam,which get into the safety containers. The maximal hydrogen amounts inboth pressurized and boiling water reactors can measure about 20,000m_(n) ³. There is also the danger that the atmospheric air in the safetycontainers (containments) will give rise to flammable mixtures, whoseuncontrolled ignition and subsequent detonation places a serious dynamiccompressive stress on the containment walls. In addition, steam andhydrogen always lead to pressure and temperature increases in theaccident atmosphere. This is particularly significant in boiling waterreactors, since their container volumes measure only about 20,000 m_(n)³, in comparison to 70,000 m_(n) ³ in pressurized water reactors.Pressure and temperature increases result in an additional static stresson the containment walls. Further, leaks owing to excess pressure cangive rise to the emission of radiotoxic substances.

Precautionary safety measures involve inerting the gas volumes withnitrogen, as has already been done for boiling water reactors. Catalyticrecombinators represent countermeasures that have been discussed andpartially implemented already. These are used to exothermallycatalytically recombine the formed hydrogen both inside and outside thelimits of inflammability, i.e., convert it into steam with thegeneration of heat. Hydrogen contents with concentrations lying withinthe limits of inflammability can also be burned off in a conventionalmanner after spark ignition. However, the resultant processes are notcontrollable, so that the system-jeopardizing reactions alreadymentioned above can arise under certain conditions.

In order to eliminate the hydrogen arising during normal operation andas the result of an accident, both thermal and catalytic recombinatorswere developed, which recombine the hydrogen with the oxygen in the airto form steam. Preference is given to catalytic systems, which operatepassively, i.e., are self-starting and need no external power supply, soas to ensure availability during an accident. Substrates used in theknown recombinators include metal plates or films as well as highlyporous granulate, on which platinum or palladium is applied as thecatalyst. Several films and granulate packets (the granulate is heldtogether in packets by wire mesh) are arranged vertically and parallelto each other in sheet casings. The hydrogen/air mixture enters into thecasing from below. The reaction starts on the catalytically coatedsurfaces. The mixture or reaction products stream over the substratesurfaces.

To date, the recombinators have made use of bilaterally coated plates orfilms and granulate packets. Their surfaces are homogenous, i.e.,covered with constant amounts of precious metal. In addition, allcatalyst elements are completely coated.

As a result, the dissipation of reaction heat from the systems isbasically problematical. It is accomplished almost exclusively viaconvection from the solid surfaces on the gases streaming past, and heatradiation to neighboring structures. However, excessive hydrogen amountscan cause the coated substrates to become overheated, so that theignition temperature is reached or exceeded, so that homogenousgas-phase reactions with deflagration or detonation can come about. Oneother disadvantage lies in the additional heating of the immediateenvironment of the substrates.

SUMMARY OF THE INVENTION

Therefore, the technical problem of this invention has to do withefficiently converting both small and large amounts of hydrogen with theatmospheric air present in the safety containers in a controlled fashionwithin a broad concentration range, and routing away the reaction heatarising in the process to such an extent that the respective ignitiontemperature is not reached in the present mixture.

The technical problem described above is resolved by a catalyst elementfor a recombinator for eliminating hydrogen from accident atmospheres,which has a flat basic body arranged inside the flow passage area of therecombinator, wherein the surface of the basic body streamed over by theaccident atmosphere is covered with varying coverage densities withcatalyst material. In this case, it was recognized according to theinvention that combining coated with uncoated or more or less coatedareas affects both the reaction rates for hydrogen conversion and thecooling of the catalytic substrate. This is because the reaction heat isrelayed into the uncoated areas via the heat conduction inside the basicbody, and there passed by convection to the as yet unreacted coolergases of the overflowing gas mixture. As a result, the level of hydrogenconversion can be suitably adjusted, wherein the large amounts of heatthat come about during recombination are advantageously limited to alevel that prevents the gas mixture of the accident atmosphere fromigniting.

The basic body can essentially have any shape desired. However, thebasic body is preferably designed as a plate or film, so that the gasmixture streaming over the surface of the basic body flows over a longerarea in the coating specially fitted with catalyst material.

The basic body can essentially be at least partially covered by catalystmaterial on all sides, so that the entire surface of the basic body isoptimally adjusted to the conversion of hydrogen. In another embodimentof this invention, the basic body has at least one uncoated and at leastone coated side. Therefore, the uncoated side of the basic body can becompletely used for dissipating the heat generated by the recombination.This is done on the one hand through heat radiation, and on the other byconvection, i.e., by releasing the heat to the gas mixture streaming by.

In a particularly preferred embodiment of this invention, the coveragedensity with catalyst material on the surface of the basic bodyincreases in the prescribed overflow direction. For this reason, thecoverage density with catalyst material is at first slight as the flowstreams over the surface of the basic body, since the share of hydrogenin the gas mixture is high, and the object is to keep down the level ofhydrogen conversion to prevent excessive heat generation. As the flowcontinues to stream, the amount of catalyst on the surface rises toincrease activity, since the share of hydrogen in the gas mixture tapersoff over the running length, and hence the danger of ignition decreasestoo.

In this case, the surface coverage density also preferably variescontinuously, wherein the surface of the basic body has coated sectionsand uncoated sections in another preferred embodiment of this invention.These sections are preferably strips, wherein the strips can be alignedboth transverse and lengthwise to the overflow direction. Anothervariation of coverage density is achieved by varying the width of thestrips in the overflow direction, or by varying the coverage densitywith neighboring catalyst material coated strips. In addition, thestrips aligned along the overflow direction can have a varying,preferably rising coverage density with catalyst material in thelongitudinal direction.

As evident from the different embodiments of this invention presentedabove, the underlying principle of a varying coverage density withcatalyst material can be configured in numerous ways.

In addition, it is also possible to provide numerous strip-shaped basicbodies, which are arranged in the flow passage area of the recombinator.These strip-shaped basic bodies can here run along or transverse to theflow direction, wherein the heights and/or coverage density withcatalyst material can vary in the strip-shaped basic bodies. While thiseliminates a continuous surface along which the gas mixture can flow,the advantage is that areas in which the gas mixture mixes and/orsettles come about in the gaps between the strip-shaped basic bodies,thereby resulting in a heat exchange and balancing of hydrogenconcentration in the gas mixture.

The above components and those claimed and described in the embodimentsto be used according to the invention are subject to no particularexceptional conditions relative to size, shape, material selection andtechnical concept, so that the selection criteria known in the area ofapplication can be fully applied. Other details, features and advantagesof the object of the invention arise from the ensuing description of theaccompanying drawings, which depict preferred embodiments of thecatalyst element according to the invention as an example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of a catalyst element according to theinvention with continuously varying coverage density of catalystmaterial, uncoated in the inflow area.

FIG. 2 shows a second embodiment of a catalyst element according to theinvention with a strip-shaped surface coated with catalyst materialrunning transverse to the direction of flow.

FIG. 3 shows a third embodiment of a catalyst element according to theinvention with a strip-shaped coating with catalyst material runningtransverse to the direction of flow, wherein the coverage density of thestrips increases in the direction of flow.

FIG. 4 shows a fourth embodiment of a catalyst element according to theinvention with coated strips aligned along the overflow direction.

FIG. 5 shows a fifth embodiment of a catalyst element according to theinvention with numerous strip-shaped basic bodies coated with catalystmaterial.

FIG. 6 shows a sixth embodiment of a catalyst element according to theinvention with numerous strip-shaped basic bodies whose surfaces aresectionally coated with catalyst material.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments relating to coatings for the surface of flat basic bodieswill be described below. The arrows indicate the preferred directions inwhich the stream flows over the basic body. Double arrows indicate thatboth directions of flow are possible. However, in the case ofnon-homogenous coatings, only one overflow direction, namely in thedirection of greater coverage density, is provided, since the hydrogenconcentration in this direction tapers due to continuing recombination.

FIG. 1 shows the surface of a plate or film 2, which is uncoated in theinlet area, with an increasing amount of catalyst material 3 in thedirection of flow. In this case, a slight coverage density is initiallyused, since the share of hydrogen is high, and the principle ofnon-ignition through low reaction rates is to be observed. As the streamflows over, the catalyst amount increases in stages or continuously upto a maximal value at the outlet. A residual portion of the dilutedmixture can still be decomposed there without an explosion despitehigher temperatures, since the share of inerted gas constituents steamand nitrogen increases owing to the increasing oxygen and hydrogenconsumption.

FIG. 2 shows a strip-coated plate or film 2. The height of the coatedand uncoated strips 4 and 6 is adjusted to the desired reaction level,and can also be varied over the running length of the surface. On theuncoated strip 6, a portion of the reaction heat from the precedingstrip 4 can be released into the substrate and on the surface. Inaddition, the free strips 6 are used for mixing the reacted andunreacted portions of the mixture. The back side of the depicted plateor film 2 can be coated in the same manner, or be completely uncoated.

FIG. 3 also shows a strip-coated plate or film. The amount of coating onthe strips 4 increases with the running length in the overflowdirection.

The embodiment on FIG. 4 shows coated strips 4 aligned in the directionof flow, in whose uncoated gaps, strips 6, a portion of the reactionheat can flow. The coverage density of the strips 4 can here be constantover the running length, or increase with the running length. Along theflow path, the already reacted gases, which contain both hydrogen andnitrogen, mix increasingly with the hydrogen-containing gas routed overthe cooling surfaces of the strips 6. Due to the changing concentrationswith higher steam contents and lower oxygen contents, the ignitabilityof the mixture is subject to targeted reductions over the runninglength.

FIG. 5 shows bilaterally or unilaterally coated plate or film strips 8,whose height is freely selectable to reflect the desired reaction levelper plate, and can therefore be optimized accordingly. For example, ifthe heights are reduced down to plate or film thickness, they approachthe thickness of a “square” wire, i.e., the catalyst elements thenconsist only of adjacent, parallel thin structures. If the samearrangement with a circular cross section were to be additionallyselected perpendicular thereto, a network would result. The height ofthe gaps is used to fix the size of the mixing and cooling zones. Thesegaps can also accommodate coolers to dissipate heat and avoidoverheating of respectively ensuing catalytically active strips. Foreach of these structures, it must be ensured that overheating can beprecluded at higher hydrogen contents.

FIG. 6 shows a division of coated plate or film strips 8 depicted onFIG. 5. The reaction on the surfaces and heat release through thermalconduction and convection can be controlled over the width of thecoating in such a way that overheating cannot take place, and henceignition temperatures cannot be reached or exceeded. In addition to theembodiment shown on FIG. 6, the coated and uncoated strips ofneighboring plate or film strips can be offset relative to each other.

1. A recombinator for eliminating hydrogen from an accident atmospherecomprising a catalyst element having a flat basic body, the flat basicbody being arranged inside an area of flow through the recombinator, asurface of the flat body over which the accident atmosphere flows has avarying coverage density of catalyst material, the coverage densityincreases in the prescribed overflow direction.
 2. The recombinatoraccording to claim 1, wherein the coverage density of the catalystmaterial varies continuously.
 3. The recombinator according to claim 2,wherein a front area of the flat basic body in the direction of flow isuncoated.
 4. The recombinator according to claim 2, wherein the surfaceof the flat basic body has coated sections and uncoated sections.
 5. Therecombinator according to claim 2, wherein a plurality of strip-shapedbasic bodies are arranged in the area of flow through the recombinator.6. The recombinator according to claim 1, wherein the surface of theflat basic body has coated sections and uncoated sections.
 7. Therecombinator according to claim 6, wherein the surface of the flat basicbody has strips comprising coated strips and uncoated strips.
 8. Therecombinator according to claim 7, wherein the strips run transverse tothe overflow direction.
 9. The recombinator according to claim 8,wherein the strips have a width which varies in the overflow direction.10. The recombinator according to claim 9, wherein the coverage densityof the catalyst material of neighboring coated strips varies.
 11. Therecombinator according to claim 8, wherein the coverage density of thecatalyst material of neighboring coated strips varies.
 12. Therecombinator according to claim 7, wherein the coverage density of thecatalyst material of neighboring coated strips varies.
 13. Therecombinator according to claim 7, wherein the strips run along theoverflow direction.
 14. The recombinator according to claim 13, whereinthe coated strips have a varying coverage density of catalyst materialin the longitudinal direction.
 15. The recombinator according to claim6, wherein a plurality of strip-shaped basic bodies are arranged in thearea of flow through the recombinator.
 16. The recombinator according toclaim 1, wherein a plurality of strip-shaped basic bodies are arrangedin the area of flow through the recombinator.
 17. The recombinatoraccording to claim 16, wherein the strip-shaped basic bodies run alongor transverse to the direction of flow.
 18. The recombinator accordingto claim 17, wherein at least one of the heights and the coveragedensities of the catalyst material of the strip-shaped basic bodiesvaries.
 19. The recombinator according to claim 16, wherein at least oneof the heights and the coverage densities of the catalyst material ofthe strip-shaped basic bodies varies.